Thin film secondary batteries comprising a Li-containing transition metal oxide such as lithium cobaltate or a Li-containing phosphate compound such as lithium phosphate are capable of charging and discharging, thin in thickness, and light in weight. Taking advantage of the features, the thin film secondary batteries comprising such a Li-containing compound are used in combination with various kinds of electric devices such as a thin film solar cell, a thin-film thermoelectric device, and a wireless charging device. The batteries are used as a power source for a credit card as well. The demand for the thin film secondary batteries is growing rapidly.
For the formation of a Li-containing oxide thin film, a sputtering method is preferably used in which a sputtering target composed of the same raw material as the film to be sputtered. The sputtering method is advantageous in that conditions for the film formation are easily adjusted, and that the film can be easily formed on a substrate of large size.
In general, the sputtering target used for a sputtering method is used as a target assembly in which the sputtering target is bonded to a backing plate by a bonding material such as a solder. The backing plate is referred to as a support. Further, bonding using a brazing material such as a solder are referred to as brazing.
FIGS. 1 and 2 illustrate schematic diagrams of a typical target assembly 21. FIG. 1 is a plan view of a target assembly. FIG. 2 is a longitudinal sectional view of FIG. 1 enlarged at A-A line. A plurality of sputtering targets 24A to 24D are arranged in FIGS. 1 and 2. This is because the area size of the sputtering target tends to increase with increasing demand for film formation on a large substrate by the sputtering method while there are certain kinds of sputtering target which are difficult to be increased in size.
The target assembly 21 illustrated in FIG. 1 comprises four pieces of the sputtering targets 24A to 24D, a backing plate 23 to fix (support) the targets, bonding material 31A to 31C for bonding a plurality of the sputtering targets 24A to 24D and the backing plate 23. Between the plurality of sputtering targets 24A to 24D, gap T is arranged for the purpose of preventing adjacent sputtering targets from contacting and chipping each other due to bending of the backing plate. On the backside (on the side of the bonding material 31A) of the gap T between the plurality of sputtering targets 24A to 24D, lining member 25 is disposed as needed so as to close the gap T. Between the sputtering targets 24A to gap T. 24D and the backing plate 23, spacers 32 are arranged so as to secure a uniform The lining member is also referred to as a stiffening plate.
Among them, the backing plate 23 is used for the purposes of supporting a sputtering targets 24A to 24D as described above in addition to cooling the sputtering targets 24A to 24D which are heated during the film formation. The backing plate 23 is thus generally made of metal materials such as pure Cu, a Cu alloy, pure Al, an Al-made alloy, or pure Ti, having a high thermal conductivity. The sputtering targets 24A to 24D mounted on the backing plate 23 are composed of a metal material in accordance with constituent of a thin film to be formed. Further, generally used as the bonding material 31A to 31C for bonding sputtering targets 24A to 24D and the backing plate 23 is a metal such as a In-based or a Sn-based solder having excellent thermal conductivity, electrical conductivity and a low melting point, or an elastic material such as an elastomer.
The target assembly 21 is produced by disposing the bonding material 31A to 31C between the sputtering targets 24A to 24D which are obtained by machining a sintered body and the backing plate 23, then bonding the sputtering targets and the backing plate by melting the bonding material by way of a heat treatment (bonding process), followed by cooling down to room temperature without changing the arrangement.
However, when the sputtering target and the backing plate are made of different materials, there is a difference in thermal expansion coefficients. Due to the difference in shrinkage after thermal expansion of the sputtering target and the backing plate, and stress caused by solidification shrinkage of the bonding material, which are arisen from a heating treatment and a cooling process, warping is generated in the target assembly. The warping generated in the target assembly causes problems such as cracking of a sputtering target or cracking a warped target assembly which is difficult to be attached to the sputtering apparatus. In order to suppress such warping due to the difference in thermal expansion between a sputtering target and a backing plate, a target assembly is flattened after bonding by being generally subjected to a correction after cooling to a room temperature or restriction in the course of cooling to a room temperature. For the correction, there is a method of, for example, applying mechanical stress in the direction to offset the warpage generated in the assembly. Further, for the restriction, there is a method of, for example, putting a weight or a clamp. Once a target assembly is warped, however, residual stress is left in the target assembly even after the correction and flattening. The residual stress is liable to cause recurrence of the warping during sputtering or at a stage close to the life end of the target, inducing cracking of the target and generation of particles. As a result, the deposition yield is significantly decreased.
It is deeply concerned for the Li-containing oxide target assembly according to the present invention that the difference in thermal expansion coefficient between the Al-based or Cu-based backing plate and the Li-containing oxide sputtering target causes warping and cracking. However, no technology has been proposed so far to solving for the problem.
Patent Document 1, for example, discloses a Li-containing transition metal oxide target having high density as well as a properly-controlled average crystal grain diameter, by which it is possible to stably obtain a sputtered film with superior homogeneity and suppressed variation in terms of composition.
In the meantime, Patent Document 2 is exemplified as a technology for preventing warping and cracking in a target assembly other than those having a Li-containing oxide target.
Patent Document 2 is particularly directed to a ceramic target, and related to a technology to effectively reduce or eliminate warping and cracking of a ceramic target in the course of bonding to a backing plate or during sputtering. The invention of Patent Document 2 was made in consideration that “cracking may occur when mechanical stress more than material strength is applied to the brittle material such as a ceramic, in particular, in a process of mechanical correction or deformation.” Finding a peculiar phenomenon that there is a certain direction along which cracking is likely to occur in the course of manufacturing process of the ceramic target, the inventors controlled the grinding direction of machining of the ceramic material.